Tomatoes (Solanum spp.) exhibit sympodial growth. This is a pattern of growth wherein the apical or terminal bud dies or ends in an inflorescence, and growth (sympodial shoots) continues from axillary or lateral buds. What looks like the plant's main axis is actually a series of many lateral branches, each arising from the previous lateral branch. After the production of some leaves by the shoot apical meristem (SAM), the growth of the primary shoot is terminated by the initiation of the first inflorescence, which is displaced from its terminal position by activation of the meristem at the axil of the last initiated leaf. The latter so-called sympodial meristem (SYM) continues shoot growth, carrying up the subtending leaf until it occupies a position above the inflorescence, which then develops laterally. The SYM undergoes a vegetative phase—producing most often three leaves—then initiates the second inflorescence, which is once again displaced laterally by the active outgrowth of the next SYM. The process is indefinitely reiterated and growth is thus indeterminate. The shoot section between two successive inflorescences is called the sympodium, and the number of leaf nodes per sympodium is referred to as the sympodial index (spi).
Thus, vegetative and reproductive phases alternate regularly during sympodial growth in tomato. In wild-type ‘indeterminate’ plants, inflorescences are separated by three vegetative nodes. As a result of this pattern of growth, a distinctive feature of tomato is its spi value. All of the red-fruited species such as S. lycopersicum have a mean spi of 3, whilst all green-fruited spp., including S. pennellii, have an average spi of 2. The spi is therefore related to fruit color and is species-specific.
Of all tomato species, S. lycopersicum (formerly L. esculentum) is the only commercially valuable species, due to its appealing and tasty fruits.